Sexual transmission is one of the major route of transmission of HIV-1 in adults. HIV-1 has been detected in semen of infected subjects. The overall objective of the proposal is to determine the role of phenotypic and genotypic properties of HIV-1 variants in semen on transmission of HIV-1. Very little information is available on the biologic and genetic characterization of HIV-1 in semen. Our hypothesis is that the distribution of HIV-1 quasispecies in blood is different than that is present in semen. Furthermore, successful transmission of HIV-1 is a result of selective transmission of specific genotype of HIV-1 present in semen. Specific aims of the study: a) To compare the genotype and phenotypes of HIV-1 in peripheral blood mononuclear cells (PBMC) and semen. b) To compare genotypes and phenotypes of HIV-1 in semen of transmitter and PBMC of recipients at the time of seroconversion. Semen and PBMC samples from the same visit will be available from a significant number of 700 seropositive subjects enrolled in the Multicenter AIDS Cohort Study (MACS) at Pittsburgh and Baltimore. In addition, 14-18 transmitter-recipient pairs from whom semen from transmitter and PBMC from recipient at the time of seroconversion will be available for this study. For genotypic characterization, V3-V5 of the HIV-1 gp12O region from uncultured semen and PBMC will be analyzed by PCR amplification of HIV-1 DNA followed by cloning and sequencing in an automated sequencer. Phenotypic characterization will be performed by measuring growth in primary macrophages, syncytia induction and virus neutralization by patient's serum. Through our previous publications and preliminary results we have demonstrated expertise in all of these techniques proposed in this application. information on the biologic and genetic characteristics of HIV-1 that are sexually transmitted will be extremely important in elucidating the mechanism of sexual transmission of HIV-1. Since potential AIDS vaccines should induce immune response against the transmitted virus, knowledge of the characteristics of transmitted HIV-1 will help us to design vaccines that are against such strains of HIV-1.